The control of pituitary hormone secretion has been studied by analysis of pituitary receptors and/or responses of regulatory factors including gonadotropin releasing hormone (GnRH), angiotension II (AII) and corticotropin releasing factor (CRF). In the rat, pituitary GnRH receptors were shown to be regulated directly by GnRH and its agonists, and to provide an index of endogenous GnRH secretion. GnRH receptors and actions were studied in gonadotrophs purified from collagenase-dispersed pituitary cells by unit gravity sedimentation and centrifugal elutriation, the ac-latter method being of particular value for preparing fractions enriched in lactotrophs and gonadotrophs. Synthetic GnRH agonists were bound by cultured gonadotrophs in direct proportion to their biological potencies on LH release, and resistance to degradation was a minor factor at the pituitary level. Upregulation of receptors by GnRH agonists was also demonstrated in cultured pituitary cells. LH responses and desensitization to GnRH were also analyzed in perifused pituitary cells,and further evidence for the role of arachidonic acid metabolites in GnRH action was obtained. Finally, All receptors were identified in th anterior pituitary and localized to lactotrophs, which were shown to respond to All with increased prolactin secretion.